In most instances when a tractor trailer dump truck is to be dumped, it is at an off-highway location where the terrain is irregular. Because of the length of the tractor trailer, even though the tractor may be on substantially level ground, the rear wheels of the trailer may not be. If the driver is not aware of this condition and raises the bed of the trailer, the load may shift to the low side. The trailer and the tractor can be turned over by this off-center load. When this occurs, it not only disrupts the delivery of the material in the trailer but causes extensive damage to the tractor and trailer. In addition, substantial costs are incurred in uprighting the tractor and trailer and also insurance costs are higher because of the damage encountered.
Many attempts have been made to provide devices on the tractor trailers to indicate when the vehicle or the load is off-center by a predetermined amount. One of these is shown in U.S. Pat. No. 3,921,128 to Snead, entitled "Truck Roll Warning System" which has a sensing device mounted on the dump bed which is responsive to a predetermined amount of tilt to cause the dump bed to be lowered and a warning signal to be provided. U.S. Pat. No. 4,145,682 to Cook, entitled "Semi-Dump Truck level Indicator" is directed to a level sensing device mounted on a dump bed having mercury switches which are responsive to a tilt either to the left or right and give an indication of the direction of tilt once a predetermined amount of tilt has occurred. Another tilt indicating device is shown in U.S. Pat. No. 4,278,854 to Krause, entitled "Tilt Indicating Device" wherein a float in a body of liquid will tilt in response to the tilt of the vehicle and complete a circuit to indicate when a predetermined amount of tilt has occurred. U.S. Pat. No. 3,464,755 to Brown, entitled "Dump Trailer Safety Device" provides a dump valve override which senses when the tilt is beyond a predetermined angle and overrides the dump valve. U.S. Pat. No. 3,657,695 to Birmingham, entitled "Leveling Indicator" discloses a level indicating device for mounting on a trailer which can be viewed from the vehicle while backing up the trailer and will give an indication as to both lateral and longitudinal tilt when it exceeds a predetermined amount. U.S. Pat. No. 4,349,809 to Tomes, entitled "Offroad Vehicle Inclinometer and Alarm" will provide an instantaneous and continuous indication of tilt of an offroad vehicle as that vehicle is being driven across the terrain. However, it is not intended for use on a tractor trailer. Thus, the deficiency of all of the devices which are intended for tractor trailers is that there is no continuous reading of the degree of tilt of the bed and by the time the predetermined angle is reached, the bed may have been raised so high that it is too late to prevent it from tilting over.
The following patents are directed to devices for automatically leveling the loads: U.S. Pat. No. 3,041,111 to Wyrick, Jr., entitled "Automatic Load Leveling System"; U.S. Pat. No. 3,512,836 to Polich, Jr., entitled "Lateral Stabilizing Means for a Semi-Trailer Dump Body"; and U.S. Pat. No. 3,640,578 to Finney, entitled "Leveling System for Semi End Dump Trucks". While these devices are satisfactory to their intended purpose, they are very costly and therefore not normally incorporated in the conventional tractor trailer.